Error analysis T. A. Herring R. W. King M. A. Floyd Massachusetts Institute of Technology GPS Data Processing and Analysis with GAMIT/GLOBK/TRACK UNAVCO

Error analysis

T. A. Herring R. W. King M. A. FloydMassachusetts Institute of Technology

GPS Data Processing and Analysis with GAMIT/GLOBK/TRACKUNAVCO Headquarters, Boulder, Colorado

10–14 August 2015

Material from T. A. Herring, R. W. King, M. A. Floyd (MIT) and S. C. McClusky (now ANU)

Basic error analysis 2

Issues in GPS Error Analysis

• What are the sources of the errors ?

• How much of the error can we remove by better modeling ?

• Do we have enough information to infer the uncertainties from the data ?

• What mathematical tools can we use to represent the errors and uncertainties ?

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Determining the Uncertainties of GPS Parameter Estimates

• Rigorous estimate of uncertainties requires full knowledge of the error spectrum—both temporal and spatial correlations (never possible)

• Sufficient approximations are often available by examining time series (phase and/or position) and reweighting data

• Whatever the assumed error model and tools used to implement it, external validation is important

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Tools for Error Analysis in GAMIT/GLOBK

• GAMIT: AUTCLN reweight = Y (default) uses phase rms from postfit edit to reweight data with constant + elevation-dependent terms

• GLOBK– rename ( eq_file) _XPS or _XCL to remove outliers– sig_neu adds white noise by station and span; best way to “rescale” the random noise

component; a large value can also substitute for _XPS/_XCL renames for removing outliers– mar_neu adds random-walk noise: principal method for controlling velocity uncertainties – In the gdl files, can rescale variances of an entire h-file: useful when combining solutions

from with different sampling rates or from different programs (Bernese, GIPSY)• Utilities

– tsview and tsfit can generate _XPS commands graphically or automatically– grw and vrw can generate sig_neu commands with a few key strokes– FOGMEx (“realistic sigma”) algorithm implemented in tsview (MATLAB) and tsfit/ensum;

sh_gen_stats generates mar_neu commands for globk based on the noise estimates– sh_plotvel (GMT) allows setting of confidence level of error ellipses– sh_tshist and sh_velhist (GMT) can be used to generate histograms of time series and velocities.

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Sources of Error

• Signal propagation effects– Receiver noise– Ionospheric effects– Signal scattering ( antenna phase center / multipath ) – Atmospheric delay (mainly water vapor)

• Unmodeled motions of the station– Monument instability– Loading of the crust by atmosphere, oceans, and surface

water• Unmodeled motions of the satellites

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Fixed antennas

Walls

Poles

Reinforced concrete pillars

Deep-bracing

http://pbo.unavco.org/instruments/gps/monumentation

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Time series characteristics


Time series componentsobservedposition

(linear)velocity term

initialposition

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observedposition


annual periodsinusoid

initialposition

Time series components

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observedposition



semi-annualperiod sinusoid

initialposition

seasonal term


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observedposition



semi-annualperiod sinusoid

initialposition

seasonal termε = 3 mm white noise


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“White” noise

• Time-independent (uncorrelated)• Magnitude has continuous probability function, e.g. Gaussian

distribution• Direction is uniformly random

“True” displacement per time stepIndependent (“white”) noise errorObserved displacement after time step t (v = d/t)

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“Colored” noise

• Time-dependent (correlated): power-law, first-order Gauss-Markov, etc

• Convergence to “true” velocity is slower than with white noise, i.e. velocity uncertainty is larger

“True” displacement per time stepCorrelated (“colored”) noise error*Observed displacement after time step t (v = d/t)* example is “random walk” (time-integrated white noise)

• Must be taken into account to produce more “realistic” velocities

This is statistical and still does not account for all other (unmodeled) errors elsewhere in the GPS system

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Annual signals from atmospheric and hydrological loading, monument translation and tilt, and antenna temperature sensitivity are common in GPS time series

Velocity Errors due to Seasonal Signals in Continuous Time Series

Theoretical analysis of a continuous time series by Blewitt and Lavallee [2002, 2003]

Top: Bias in velocity from a 1mm sinusoidal signal in-phase and with a 90-degree lag with respect to the start of the data span

Bottom: Maximum and rms velocity bias over all phase angles– The minimum bias is NOT obtained with

continuous data spanning an even number of years

– The bias becomes small after 3.5 years of observation

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Characterizing the Noise in Daily Position Estimates

Note temporal correlations of 30-100 days and seasonal terms

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Figure 5 from Williams et al [2004]: Power spectrum for common-mode error in the SOPAC regional SCIGN analysis. Lines are best-fit WN + FN models (solid=mean ampl; dashed=MLE)

Note lack of taper and misfit for periods > 1 yr

Spectral Analysis of the Time Series to Estimate an Error Model

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Summary of Spectral Analysis Approach

• Power law: slope of line fit to spectrum– 0 = white noise– -1 = flicker noise– -2 = random walk

• Non-integer spectral index (e.g. “fraction white noise” 1 > k > -1 )

• Good discussion in Williams [2003]

• Problems: – Computationally intensive– No model captures reliably the lowest-frequency part of the spectrum

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CATS (Williams, 2008)

• Create and Analyze Time Series• Maximum likelihood estimator for chosen

model– Initial position and velocity– Seasonal cycles (sum of periodic terms) [optional]– Exponent of power law noise model

• Requires some linear algebra libraries (BLAS and LAPACK) to be installed on computer (common nowadays, but check!)

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Hector (Bos et al., 2013)

• Much the same as CATS but faster algorithm• Maximum likelihood estimator for chosen model– Initial position and velocity– Seasonal cycles (sum of periodic terms) [optional]– Exponent of power law noise model– Also

• Requires ATLAS linear algebra libraries to be installed on computer

• Linux package available but tricky to install from source due to ATLAS requirement

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sh_cats/sh_hector

• Scripts to aid batch processing of time series with CATS or Hector

• Requires CATS and/or Hector to be pre-installed

• Outputs– Velocities in “.vel”-file format– Equivalent random walk magnitudes in “mar_neu”

commands for sourcing in globk command file• Can take a long time!2015/08/11


White noise vs flicker noise from Mao et al. [1999] spectral analysis of 23 global stations

Short-cut (Mao et al, 1998): Use white noise statistics ( wrms) to predict the flicker noise

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“Realistic Sigma” Algorithm for Velocity Uncertainties

• Motivation: computational efficiency, handle time series with varying lengths and data gaps; obtain a model that can be used in globk

• Concept: The departure from a white-noise (sqrt n) reduction in noise with averaging provides a measure of correlated noise.

• Implementation:– Fit the values of chi2 vs averaging time to the exponential function

expected for a first-order Gauss-Markov (FOGM) process (amplitude, correlation time)

– Use the chi2 value for infinite averaging time predicted from this model to scale the white-noise sigma estimates from the original fit

– and/or– Fit the values to a FOGM with infinite averaging time (i.e., random

walk) and use these estimates as input to globk (mar_neu command)

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Extrapolated variance (FOGMEx)• For independent noise, variance ∝

1/√Ndata

• For temporally correlated noise, variance (or 𝜒2/d.o.f.) of data increases with increasing window size

• Extrapolation to “infinite time” can be achieved by fitting an asymptotic function to RMS as a function of time window– 𝜒2/d.o.f. ∝ e−𝜎𝜏

• Asymptotic value is good estimate of long-term variance factor

• Use “real_sigma” option in tsfit

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Basic error analysis 25Yellow: Daily (raw) Blue: 7-day averages

Understanding the FOGMEx algorithm: Effect of averaging on time-series noise

Note the dominance of correlated errors and unrealistic rate uncertainties with a white noise assumption: .01 mm/yr N,E.04 mm/yr U

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Same site, East component ( daily wrms 0.9 mm nrms 0.5 )

64-d avgwrms 0.7 mmnrms 2.0



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Red lines show the 68% probability bounds of the velocity based on the results of applying the algorithm.

Using TSVIEW to compute and display the “realistic-sigma” results

Note rate uncertainties with the “realistic-sigma” algorithm :

0.09 mm/yr N0.13 mm/yr E0.13 mm/yr U

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Comparison of estimated velocity uncertainties using spectral analysis (CATS) and Gauss-Markov fitting of averages (FOGMEx)

Plot courtesy E. Calais

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Summary of Practical Approaches

• White noise + flicker noise (+ random walk) to model the spectrum [Williams et al., 2004]

• White noise as a proxy for flicker noise [Mao et al., 1999]• Random walk to model to model an exponential spectrum [Herring “FOGMEx”

algorithm for velocities]• “Eyeball” white noise + random walk for non-continuous data______________________________________ • Only the last two can be applied in GLOBK for velocity estimation• All approaches require common sense and verification

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External validation of velocity uncertainties by comparing with a model - Simple case: assume no strain within a geologically rigid block

GMT plot at 70% confidence

17 sites in central Macedonia: 4-5 velocities pierce error ellipses

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.. same solution plotted with 95% confidence ellipses

1-2 of 17 velocities pierce error ellipses

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Basic error analysis 32McCaffrey et al. 2007

External validation of velocity uncertainties by comparing with a model - a more complex case of a large network in the Cascadia subduction zone

Colors show slipping and locked portions of the subducting slab where the surface velocities are highly sensitive to the model; area to the east is slowly deforming and insensitive to the details of the model

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Velocities and 70% error ellipses for 300 sites observed by continuous and survey-mode GPS 1991-2004

Test area (next slide) is east of 238E

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Residuals to elastic block model for 73 sites in slowly deforming region

Error ellipses are for 70% confidence: 13-17 velocities pierce their ellipse

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Cumulative histogram of normalized velocity residuals for Eastern Oregon & Washington ( 70 sites )

Noise added to position for each survey: 0.5 mm random 1.0 mm/sqrt(yr)) random walk

Solid line is theoretical for a chi distribution

PercentWithinRatio

Ratio (velocity magnitude/uncertainty)

Statistics of Velocity Residuals

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PercentWithinRatio

Same as last slide but with a smaller random-walk noise added :

0.5 mm random 0.5 mm/yr random walk

( vs 1.0 mm/sqrt(yr)) RW for ‘best’ noise model )

Note greater number of residuals in range of 1.5-2.0 sigma


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PercentWithinRatio

Same as last slide but with larger random and random-walk noise added :

2.0 mm white noise 1.5 mm/sqrt(yr)) random walk

( vs 0.5 mm WN and 1.0 mm/sqrt(yr)) RW for ‘best’ noise model )

Note smaller number of residuals in all ranges above 0.1-sigma



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Summary

• All algorithms for computing estimates of standard deviations have various problems: Fundamentally, rate standard deviations are dependent on low frequency part of noise spectrum which is poorly determined.

• Assumptions of stationarity are often not valid

• FOGMEx (“realistic sigma”) algorithm is a convenient and reliable approach to getting velocity uncertainties in globk

• Velocity residuals from a physical model, together with their uncertainties, can be used to validate the error model

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Documents

Error analysis T. A. Herring R. W. King M. A. Floyd Massachusetts Institute of Technology GPS Data Processing and Analysis with GAMIT/GLOBK/TRACK UNAVCO